Salt bath for heat treatment and rectifier therefor



Patented Feb. 12, 1946 UNITED STATES P SALT BATH FOR HEAT TREATMENT ANDRECTIFIER THEREFOR Artemas F. Holden and Haig Solakian, New

Haven, Conn.; said Solakian assignor to said Holden No Drawing.

11 Claims.

The present invention relates broadly to metallurgy and more especiallyto a salt bath for heat treatment of ferrous metals parts, and rectifiertherefor.

The present invention is more particularly an improvement upon the saltbath designated as #2 bath in Holden and Solakian Patent #2,237,- 434,patented April 8, 1941. The #2 bath of the said patent comprised silicasand, calcium fluoride, calcium oxide, sodium carbonate and sodiumfluoride. This bath absorbed iron oxide from the work which tended tomake the bath alkaline and which is undesirable. The bath of the presentinvention does not absorb iron' oxide and is inert to the ferrous metalvbeing treated, as well as to the refractory or ceramic pots in which thebath is contained. There is a slight tendency of the #2 bath of Patent#2,237,434 to react with ceramic pots.

The ceramic pot preferably used with the present salt bath is a potcomposed of aluminum oxide and aluminum silicate combinations wellknownin the art.

The present invention is a neutral high temperature bath adapted tooperate in a working range of 1750 F. to 2350 F. and is adaptable moreespecially for treating high speed steels, such as high carbon-highchrome types of steels as well as the various high speed steelsspecially referred to in the said Patent #2,237,434.

The preferred chemicals for the present bath are barium chloride,titanium dioxide, and a small percentage of silicon dioxide. Thepercentage ranges may be from 90% to 99% barium chloride; /2% to 5%titanium dioxide; and to 5% silicon dioxide. The preferred percentagesare 98% barium chloride, 1%% titanium dioxide, and /z% silicon dioxide.

A satisfactory bath may comprise 95% to 99% barium chloride and 1% to 5%titanium dioxide, the preferred bath being 98% barium chloride and 2%titanium dioxide. The omission of the silicon dioxide does not seriouslyimpair the operation of the bath, although it does remove a slightsafety factor to the life of the bath.

Another satisfactory formula for this salt bath may comprise bariumchloride, barium fluoride, calcium fluoride (alkaline earth metalhalides), chromium fluoride, and titanium dioxide, plus a small amountof silicon dioxide or sand in proportions comprising 70% to 90% bariumchloride; 3% to 7%, barium fluoride; 3% to 8% calcium fluoride; 2% to 7%chromium fluoride; and /2% to 3% titanium dioxide, plus /2% to 5%silicon dioxide (which may be omitted), and the Pre- Application August31, 1943, Serial No. 500,644

ferred bath of these ingredients comprises 84 barium chloride; 5% bariumfluoride; 5% calcium fluoride; 4% chromium fluoride; and 1% titaniumdioxide, plus a% silicon dioxide. The silicon dioxide may be omitted, inwhich case the barium chloride is increased to 85%.

In the operation of the baths above specifically set forth, the bariumchloride when heated to the temperature specified, namely, 1750 F. to2350 .F., tends to break down into barium oxide plus chlorine which goesoil as a gas. The presence of barium oxide in the bath tends to make thebath alkaline which is undesirable. The titanium dioxide reacts andcombines with the barium oxide to form barium titanate. The formulabeing BaClz- Ba0+C1; and BaO+TiOz- BaTiOa. The barium titanate isneutral and at the lower temperatures the barium titanate will form asludge and settle at the bottom of the pot. Thus it will be seen that inthe operation of the bath the barium chloride is slowly decomposed andthe titanium dioxide is slowly combined into a differ.- ent inertchemical, namely, barium titanate.

During the working of the bath the percentages of the ingredients getout of balance, and the bath, instead of remaining neutral, becomesalkaline and gradually increases in alkalinity. An alkaline bath, athigh temperatures, will decarburize steel parts, and this is veryundesirable. In order to avoid this and to bring the bath back toneutral balance. the method of introducing into the bath the chemicalsof the bath in abnormal percentage relationship, has been suc cessfullyemployed. We have designated such a mixture as a rectifier. Therectifier in the present case preferably comprises barium chloride,titanium dioxide and silicon dioxide, the range of percentages being 4%to 25% barium chloride, to 95% titanium dioxide, 1% to 15% silicondioxide, the preferred percentage for the rectifier being 20% bariumchloride, titanium dioxide, and 10% silicon dioxide.

A satisfactory rectifier may be obtained by omitting the silicondioxide, under which condition the percentages may be 5% to 25% bariumchloride, to 95% titanium dioxide, the preferred percentages being 20%barium chloride, and titanium dioxide.

With reference to the bath including as ingredients the fluoridesspecified, namely, barium, calcium, and chromium fluoride, the aboverectiflers are satisfactory to maintain neutrality of the bath forthe'reason that the fluorides decompose extremely slowly and may bereplenished by adding to the bath the ingredients of the original bathwhich is necessary to replace the material which is taken from the potby the work as it is lifted out of the not after heat treatment..However, in order to maintain the fluidity of the bath as well asneutrality, it may be desirable to add small percentages of the calciumfiuoride and chromium fluoride to the rectifier. Both of these fluoridestend to lower the melting point 01' the bath and to overcome alkalinity.Where calcium fluoride and chromium fluoride are added, the rectifierbecomes titanium dioxide, calcium fluoride and chromiumfiuoride in per--centages of 60% to 90% titanium dioxide, to 25% calcium fluoride, 2% to15% chromium fiuoride, the preferred percentages being 85% titaniumdioxide, calcium fluoride, 5% chrc= mium fluoride.

The condition of the bath for neutrality can be checked by taking a 1gram sample, dissolving the same in 100 cubic centimeters of water,filtering and then titrating the filtrate with .1% normal hydrochloricacid, using methyl red as an indicator; the pink color being the endpoint.

If the number of cubic centimeters of hydrochloric acid required tobring the end point exceeds two cubic centimeters, then A% of 1% ofrectifier needs to be added. In other words, if the pot contains 100lbs. of salt bath, then 4 ounces of the rectifier should be added tobring the bath back to normal neutrality.

Rectification of the bath is not frequently required and may not benecessary more often than at intervals of thirty-six hours with acontinuously operated bath.

The salt baths herein specified as comprising the present invention arenormally neutral in character, operate at temperatures between 1750 F.and 2350 F., and comprise materials which are soluble in. water to amuch. greater degree than the #2 bath specified in. Patent 2,237,434.Such material of the present baths .as adhere to the work is quicklywashed. away in #3A or. #33 baths specified in said Patent #2,237,434.

What we claim is:

1. A substantially neutral salt bath for heat treatment of ferrousmetals at temperatures from 1750" F, to 2350 F., consisting essentiallyof an alkaline earth metal chloride salt comprising the major portion ofsaid bath and tending to become an alkaline earth metal oxide at thetemperatures specified; and titanium dioxide to convert the alkalineearth metal oxide formed into a neutral alkaline earth metal titanate.

2. A salt bath for heat treatment of ferrous metals at temperatures from1750" F. to 2350 F., consisting essentially of barium chloride as amajor portion of said bath which slowly decomposes into chlorine gas andbarium oxide at the temperatures specified; titanium dioxide to form thesaid oxide into barium titanate and A% to 5% of silicon dioxide.

3. A salt bath for heat treatment of ferrous metal parts at temperaturesbetween 1750 I". to 2350 F., and comprising 95% to 99% barium chlorideand 1% to 5% titanium dioxide.

4. A salt bath for heat treatment of ferrous metal parts at temperaturesbetween 1750 F. to 2350 F., and comprising substantially 98% bariumchloride and 2% titanium dioxide.

5. A salt bath for heat treatment of ferrous metal parts at temperaturesbetween 1750 F. to 2350 F., comprising 90% to 99% barium chloride; /2%to 5% titanium dioxide; and to 5% silicon dioxide.

6. A salt bath for heat treatment of ferrous metals at temperatures offrom 1750 F. to 2350 EX,v comprising substantially 98% barium chloride;1 /2% titanium dioxide, and /2% silicon dioxide.

'7. A rectifier for a heat treatment salt'bath, in which a chloride salttends to decompose into an alkaline oxide when heated, said chloridesalt comprising more than of the bath, and a quantity of titaniumdioxide to convert said alkaline oxide into an inert neutralcomposition; said rectifier comprising 60% to 95% said titanium dioxideand 4% to 25% said chloride salt.

8. A rectifier for a heat treatment chlorid salt bath, in which achloride bath decomposes into an alkaline oxide when heated to thedecomposing temperature of said salt, said salt comprising more than 70%of said bath, and a quantity of titanium dioxide to convert said oxideinto an inert neutral composition; said rectifier comprising saidchloride salt and more than 60% of said titanium dioxide. I

9. A rectifier for a substantially neutral salt bath for heat treatmentof ferrous metals, said salt bath. comprising a chloride salt tendingwhen heated 'to decomposing temperatures to break down into chlorine gasand an alkaline oxide, said chloride salt comprising more than 70% ofsaid bath; said'rectifier comprising titanium dioxide adapted to convertsaid alkaline oxide into an inert neutral composition, said titaniumdioxide comprising more than 60% of said rectifier.

10. A rectifier for a salt bath for heat treatment of ferrous metals attemperatures from 1750 F. to 2350 F., said salt bath comprising 95% to99% barium chloride and 1% to 5% titanium dioxide; said rectifiercomprising 5% to 25% barium chloride and to 95% titanium dioxide.

11. A rectifier for a salt bath for heat treatment of ferrous metals attemperatures from 1750 F. to 2350 F., said salt bath comprisingsubstantially 98% barium chloride and 2% titanium dioxide; saidrectifier comprising substantially 20% barium chloride and titaniumdioxide.

' ARTEMAS F. HOLDEN.

HAIG SOLAKIAN.

